Electro-optic modulators are used in fiberoptic communication systems, in particular, commercial high speed, long distance digital communication systems. Electro-optic modulators convert electrical signals representing data or voice into modulated optical signals, suitable for transmission over optical media.
Techniques for modulating optical signals include amplitude modulation and phase modulation. Amplitude modulation involves modulating optical signals, such that the light emitted from an electro-optic modulator is switched between xe2x80x9cONxe2x80x9d and xe2x80x9cOFFxe2x80x9d states. A Mach-Zehnder interferometer is one example of a push-pull modulator that can be used to implement amplitude modulation. On the other hand, phase modulation results in the phase of optical signals being shifted a certain number of degrees. Phase modulated signals require specialized wave detectors to detect changes in phase, while amplitude modulated signals require photon detectors that detect light intensity.
One limiting factor of electro-optic modulators is the high switching voltages required for data transmissions at 10 gigabits per second (Gbit/s) and above. For push-pull modulators, the switching voltage, Vxcfx80, is the voltage swing required to modulate light between xe2x80x9cONxe2x80x9d and xe2x80x9cOFFxe2x80x9d states. Existing electrode designs require compromises among velocity, electrical/optical overlap (a measure of modulation efficiency), impedance, microwave loss, and manufacturability.
The present invention relates to an electro-optic push-pull modulator with reduced switching voltages. A reduction in switching voltage is realized through a combination of device structure and operation to cause addition of the so-called linear and quadratic electro-optic effects that result in refractive index changes in response to applied electric fields. The device structure may specify crystal axis orientation, waveguide structure, or electrode structure; the device operation may specify electric field biasing, operating wavelengths, or optical polarization. By inducing the linear and quadratic electro-optic effects to add, significant refractive index changes can be realized with a lower switching voltage, Vxcfx80.
In particular, one preferred embodiment of the invention includes a substrate, an optical waveguide structure with electro-optic properties, and an electrode structure. In a III-V semiconductor material operation at a wavelength near the absorption edge (i.e., with photons having energy near but just below the band gap energy) causes the material to exhibit refractive index changes that vary quadratically in response to electrical fields applied substantially perpendicular to it (i.e., quadratic electro-optic effect). This effect occurs in the bulk material, but it can be strengthened by using a multiple quantum well (MQW) structure in the waveguide. Furthermore, the waveguide exhibits refractive index changes that vary linearly in response to applied electrical fields (i.e., linear electro-optic effect). The linear response is determined by the crystal axis orientation of the electro-optic material and the optical polarization of propagating optical signals.
The optical waveguide branches into a first waveguide arm and a second waveguide arm. Optical signals propagate through the waveguide, splitting into the waveguide arms where they are independently modulated by varying electrical fields applied by an electrode structure.
The electrode structure is disposed about the substrate applying electrical fields to each waveguide arm, substantially perpendicular to the MQW structure. Controlled by a modulation voltage, the electrical fields induce approximately equal and opposite refractive index changes in the waveguide arms, driving push-pull modulation of the optical signals. The electrode structure also biases the electrical fields about an electrical field having a non-zero magnitude and a direction, such that the linearly-varying and quadratically-varying refractive index changes add together while still retaining push-pull operation. The modulator performance may also be enhanced by proper selection of operating wavelength of the optical signals to induce a voltage-dependent optical absorption due to the Quantum-Confined Stark Effect (QCSE), but long enough to minimize optical losses. Thus, lower switching voltages may be applied to the electrode structure inducing equal and opposite refractive index changes in the waveguide arms for push-pull operation.
Further embodiments of the invention reduce switching voltages of electro-optic push-pull modulators through combinations of device structure and operation effectively inducing refractive index changes due solely to the quadratic electro-optic effect. This invention is applicable to any push-pull modulator that uses a waveguide structure that exhibits the quadratic electro-optic effect or a combination of quadratic and linear electro-optic effects. Although the detailed description of the preferred embodiment is specific to using III-V semiconductor material for waveguide and substrate, the general principles taught here can be applied to modulators built in different materials or combinations of materials. Furthermore, this invention is applicable to a polarization independent modulator with each polarization considered separately using the principles illustrated herein.